Designers and semiconductor device manufacturers constantly strive to develop smaller devices from wafers, recognizing that circuits with smaller features generally produce greater speeds and increased packing density, therefore increased net die per wafer (numbers of usable chips produced from a standard semiconductor wafer). To meet these requirements, semiconductor manufacturers are involved in a continuous process of building new fabrication lines at each new “next generation” process node (gate length). As the critical dimensions for these devices grow smaller, greater difficulties will be experienced in patterning these features using conventional photolithography.
Conventional photolithography methods used for pattern generation involve exposing a light sensitive photoresist layer to a light source. The light from the source is modulated using a reticle, typically a chrome-on-quartz mask reticle. During processing, reticle patterns are transferred to a photoresist layer formed on a semiconductor substrate. Commonly, such pattern transfer is achieved using visible or ultraviolet light. The exposed photoresist pattern is then developed to form a pattern of photoresist on the substrate. The developed regions are then washed away and the remaining photoresist pattern used to provide an etching mask for the substrate.
One newer approach to achieving the desired critical dimensions has been to use attenuated phase shift masks and strong phase shift masks. Such masks have many useful properties. However, such masks suffer from a number of shortcomings. Phase shifting masks are very difficult to produce; and unlike binary masks, are not readily reconfigurable. Additionally, conventional phase shifting masks commonly require two or more exposures per substrate layer to obtain a desired pattern. This has the effect of lowering throughput to perhaps 40% of that achievable with a single exposure approach.
An example of such a new process technology is embodied, for example, in optical direct write process techniques. One example of such a technique is taught, by the above-referenced inventors, in U.S. patent application Ser. No. 10/825,342, entitled: “Optimized Mirror Design for Optical Direct Write”, filed on Apr. 14, 2004 and hereby incorporated by reference for all purposes.
An optical direct write system makes use of a programmable mirror array to generate photolithographically reproducible optical patterns that are projected onto a photoimageable layer. For example, an optical beam is directed onto the mirror array at an angle normal to the mirror array to produce an optical pattern. The optical pattern is then projected onto a substrate with a photoimageable layer. The reflected light pattern (i.e., reflected from the mirror array) exposes the photoimageable layer to transfer a desired pattern onto the substrate. Advantageously, the mirror array of the optical direct write system can be reconfigured by merely implementing software instructions to reconfigure the arrangement and orientation of the mirrors of the array.
In some implementations, mirror arrays are configured to generate phase shift exposure patterns. Typically, photolithographic optical settings (commonly including focus and dose, but not limited to such) and phase shifting mirror arrays are optimized for a process to produce the best process window for a given critical dimension. Commonly, a user/lithographer will optimize the process window for the smallest critical dimension to be found on the target substrate. Typically, this smallest critical dimension is associated with smallest feature desired or is associated with the smallest line pitch desired for a given process layer. The settings are optimized to generate a process window capable of faithfully reproducing the smallest feature with a desired degree of fidelity. When the settings are optimized in this way, they are generally excellent for reproducing dense line pitches or very small features. However, due to the nature of phase shift lithography, such optimized settings lose fidelity and sharpness when applied to other critical dimensions or significantly different line pitches. Thus, settings used to produce dense line pitches and small critical dimension (CD) features can be unsuitable for larger features. This is problematic because a typical semiconductor has a healthy mix of feature sizes and pitches. Thus, systems optimized for the worst case scenarios (small CD's and dense pitch patterns) are not optimized for larger features. This means that when systems optimized for dense patterns or short line pitches are used for less dense patterns unintended light scattering effects degrade the contrast and quality of the image pattern. For example, by creating periodic ghost patterns of alternating dark and light regions and causing drift in the width and position of features. Such systems must be re-optimized to image the larger features. This takes time and additional exposures and accordingly reduces throughput for the affected systems.
In view of the above difficulties, what is needed is a relatively simple and effective solution to such processing difficulties.